The Soviets were willing to do pretty much anything to get their hands on a B-29. They all but beggedfor permission to get them under the "lend-lease" act but were repeatedly denied.
There were 4 cases when we did land (Pacific based) B-29's on USSR air fields; none of the aircraft were ever returned to the U.S.
The US B-29's bombed Japan, were damaged (or had other issues that caused them to run out of fuel), and had to make emergency landings at air fields controlled by the USSR.
The air crews were (eventually) sent back to the U.S. However, the aircraft were held, inspected, flown to Moscow, completely disassembled, reassembled, eventually copied, and the USSR mass produced an exact carbon copy; the Tu-4.
Boeing B-29 Superfortress: